How to prevent untempered masonite from warping.

My artist wife typically paints on 1/4 in or 1/8 in (3 ft by 4 ft)
untempered masonite panels.
The panels are always first painted or gessoed on both sides and on
all 4 ends. They are usually not framed. Never had a problem until
several years ago.
Up until then the panels which I bought from regular lumber yards were
smooth on one side and rough textured on the other side.Now the panels
are smooth on both sides.
Now they warp in a short period.So I tried several things.
1. I glued a baltic birch plywood cross brace on the back. No luck.
2. I glued a baltic birch frame around the back plus the cross brace.
Again. no luck.
3. I added a regular frame . Again, no luck.
Any suggestions?
My wife has shifted to canvas panels. but would like to go back to
masonite.

Is there any possibility that a local lumber yard could order the other kind
of hardboard? It would be worth a phone call or two. It might also be
worth looking around on the web to see if anybody still makes or sells it.

I did a quick search. I came up with these.
http://www.dickblick.com/zz149/45 /
http://www.danielsmith.com/learn/techLeaflets/0012 /
http://painting.about.com/od/paintingforbeginners/a/hardboard.htm
There is a lot more. If you want to continue on with the search, use this;
http://www.google.com/search?as_q=masonite%2C+paint&num &hl=en&btnG=Google+Search&as_epq=&as_oq=&as_eq=doors&lr=lang_en&as_ft=i&as_filetype=&as_qdr=all&as_occt=any&as_dt=i&as_sitesearch=&as_rights=&safe=images
From what I read, the tempered hardboard has oils that canlechout and
interfere with the paints. I really don't know if that would be a problem in
terms of warping though.

For clarity, does it matter if you use tempered or untempered? I have had a
couple of old timers tell me and IIRC WoodSmith had an article on hard
board. They all state that you cannot look at hard board and tell if it is
tempered or non tempered. They both come in varying degrees of darkness or
lightness and both may or may not have a slick back. Apparently the
factory paints a stripe down the side of a stack of hardboard either red or
green. Green is nontempered, red is tempered. If your material has been
cut from one or the other it can be easily mixed up.

I have a cousin that is an artist. Similar situation, and he switched
to hollow core doors. Cuts them to size and glues in a wood filler in
the cut end. Simple to do, relatively cheap, and they're very stable.
The overall depth isn't that much greater than a stretched canvas.
R

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